What about momentum?
On one hand, sundials stop momentumed creatures, since they are not a shield.
On the other hand, momentum seems to mean that the creature is so fast and heavy, it can run through any defenses without being stopped, so it possibly could run straight through the maze, breaking the walls on it's way.
Generally, we have to be careful with defensive cards that ignore momentum, since they would make momentum useless. Why use cards meant to ignore defenses, if they don't work?
With sundials it might be fair that they ignore momentum, since the sundial itself is temporary and after sundials run out, momentum is once again useful.
We also need to be careful with non-shield defensive cards. There is a reason why there are so few of them (Flooding, Sundial). One thing is that shields are meant to have their weaknesses, eg. phase shields are temporary, bone walls are vulnerable to swarms, dissipation shields slow you down by draining your quanta, fire and thorn shields offer little or no protection, hope is vulnerable to CC. Non-shield defensive permanents might be able to prevent opponents from using this weakness, thus offering a defense without weaknesses. This might not be a case right now, since all these setups are vulnerable to PC so far, but it generally seems that Zanz intended that shields should be always the main defense, while other defensive permanents should be few and have greater weaknesses than shields. I guess no one disagrees that both flooding and sundial are now weaker than shields. (Ignore the pre-nerf sundial, since it was an obvious mistake)
Anyway, even if Maze Growth is not a broken idea right now, it might limit the future shield ideas (eg. shields that block only airborne creatures, shields that block a certain number of attacks each turn, etc.)
I'm not sure what to do about it, since it's hard to tell if this card is OP without proper testing, but personally, I'd give it an upkeep cost of 1
. Right now flooding, the only non-temporary non-shield defensive permanent has an upkeep cost, so it would seem like a good solution to discourage abusing Maze (eg. using it in rainbow, paying for it with mark only or with novas/supernovas only), while keeping it still very useful in mono and duo.